In a known manner video stream transmission over a wireless channel commonly uses inter frame prediction, also known as motion compensation. This method is used in all already deployed video compression systems and also for recent standards such as H.264 standard.
Video compression consists in predicting a block of a video frame (also called a macroblock in H.264) by translating an area of at least one reference frame using a motion vector. Many types of macroblocks exist. For example, intra predicted macroblock, called I macroblock, is predicted upon spatial neighboring macroblocks of the current frame, P macroblock uses a single motion vector associated to a reference area and B macroblock uses a bipredictive motion compensation scheme with two motion vectors associated to two reference areas.
Each decoded frame can be used for future prediction and video artifact propagation and expansion is due to the fact that frames are predicted upon previous decoded frames.
The problem is to optimize video delivery, and particularly to avoid inter frame video artifact propagation, in the case of low delay and low bandwidth wireless videophonie applications, considering the compression of the video stream over an unreliable bidirectional transmission channel.
The present invention is particularly well adapted to mobile video telephony applications, but can also be adapted to unicast video streaming. The scheme is not complex to implement (compared to ARQ schemes) and can reduce the problem of inter-frame artifact propagation which has a huge impact on perceptive video quality.
A common scheme for handling video artifact propagation is the periodic insertion of I macroblocks, as erroneous macroblocks from previous reference frames are not used in the prediction scheme. Inter frame artifact propagation is then stopped. This prediction method is not efficient in terms of compression. More often intra macroblocks are inserted, higher is the protection against inter frame artifact propagation, but lower is compression efficiency as Intra macroblock requires lots of bandwidth.
Erroneous macroblock detection can be done at receiver side by the radio decoder, which sends then a request to the encoder that will retransmit the requested part. This method may not be compatible with the low delay constraint required in video telephony applications. One key point is to keep video synchronized with real time audio (lip synchronization), thus limiting time allocated to Automatic Repeat request (ARQ) mechanisms and requiring extra bandwidth for retransmitted parts. When experiencing high bit error rate (BER), the retransmission bandwidth can be too large, the decoder may need to request retransmission more than one time.
In order to avoid using inter frame prediction upon an erroneous previous frame, another solution from the prior art is to select a reference image according to Acknowledgement/Non-Acknowledgement from the decoder as described in H.263 standard.